The development of rotation sensors which eliminate spinning-mass gyroscopes with their many moving parts, high cost, complexity, and in some cases inaccuracy and unreliability has led to many different approaches. Passive ring sensors, such as the multi-turn, single-pass fiber optic interferometer and the multi-pass, single-turn open resonator lack the sensitivity required for high accuracy. The active approaches, such as ring laser gyros, are much more promising.
The conventional ring laser gyro is based on two counter-rotating optical resonator modes. These modes split in frequency when the gyro rotates about its sensitive axes. The frequence split, which is proportional to the rotation rate and the area enclosed by the optical mode is measured by the use of optical heterodyne detection of the two frequencies. In practice, however, there is always some scattering of light from one mode into the other which causes the well-known lock-in phenomena. As a result, the two frequencies tend to be locked together and do not split for low rotation rates. These two optical modes, which occupy the same physical space, are degenerate due to injection locking and the degeneracy is not easily removed. This causes a dead band in the gyro output which may be as wide as 2,000 degrees/hour. Furthermore, even for rotation rates above the lock-in threshold, the output remains a nonlinear function of rotation rate.
Methods to circumvent lock-in have led to the development of a variety of sensors. Most approaches try to remove the degeneracy and overcome this problem by applying nonreciprocal bias to the gyro output frequency either by rotating the gyro or by inserting into the cavity an optical element such as a magnetic mirror or a Faraday cell. But large biases are required and bias drifts produce large inaccuracies. An improvement is obtained when the bias is intentionally reversed "exactly" one-half the time by mechanical or electro-optical dithering. The main drawback to the dither scheme is that the gyro passes through the lock-in region at twice the dither rate. Each time the gyro leaves lock-in it loses its memory of phase and thus, an error of a fractional count accumulates once per cycle. These errors add randomly giving an angular error which increases as the square root of elapsed time. This type of error source is particularily serious for short measurement times where the uncertainty corresponds to a large percent error in the apparent rotation rate. In all cases the use of a single bias element to circumvent lock-in have seriously compromised the inherent potential of the ring laser rotation sensor.
It must be realized that the two counter rotating modes of a conventional ring laser may be considered as four degenerate modes when polarization is taken into account. Each longitudinal mode can oscillate as any of four distinct waves, a clockwise (CW) and a counter-clockwise (CCW) traveling wave, with each having either of two arbitrary orthogonal polarizations. All four of these possible waves have identical unperturbed resonance frequencies, thereby allowing very small amounts of backscatter to accomplish mutual phase locking. It is this four-fold frequency degeneracy that causes the lock-in difficulties. If the modes had different resonance frequencies or if they did not occupy the same physical volume so that scattering could not cause lock-in, the problems inherent in those devices would be circumvented. Realization of a four-frequency or multioscillator laser gyro which circumvents the lock-in problem requires that this degeneracy be fully removed. That is, both the directional degeneracy and the polarization degeneracy must be removed. This would allow simultaneous oscillation at four different frequencies for each longitudinal cavity mode. As in the case of the biased ring laser gyro discussed above, the directional degeneracy can be removed by inserting into the cavity a non-reciprocal (direction-dependent) polarization rotator. This produces different resonant frequencies for the CW and the CCW waves. The polarization degeneracy can then be removed by inserting into the cavity a reciprocal (direction-independent) rotator. This produces different resonant frequencies for the right circular polarized wave and the left circular polarized wave which are traveling in the same direction. It is this last element which makes this ring laser gyro differ from the others. The use of these two passive bias elements in the cavity do fully remove the four-fold degeneracy and circumvent lock-in. In this context, reference to a passive element means that the element does not interact with the beam in a manner that causes gain. This does not mean that the passive element does not interact with the beam in other ways; neither does it mean that losses are not introduced.
Disadvantages of the four-frequency ring laser gyro are primarily associated with the losses produced by the polarization rotators introduced into the optical cavity and, in particular, with the nonreciprocal polarization rotator. This nonreciprocal polarization rotation may be accomplished by the use of Zeeman biasing in the gain medium, by the use of the magneto-Kerr effect in a magnetic mirror, by the use of the Faraday effect in a Faraday cell, or possibly by some other means. All of these are lossy and decrease the Q of the cavity which increases the linewidth of the passive cavity resonance. This places severe requirements on the biasing of this element. These losses also increase the pumping requirements on the active medium because the gain curve will have to be maintained above the threshold over the entire frequency band of interest.
Another technique used to prevent lock-in is to use two optical fiber waveguides as separate ring lasers with cavities spatially separated, and to cause each cavity to contain traveling waves in only one direction. The disadvantages of the fiber waveguide ring laser gyroscope are primarily associated with inherent properties of the fibers. Additionally, the nature of lasers which may be utilized as active media in these devices also cause major problems which actually prevents the sensitivity that is required for high accuracy. Components used to selectively restrict the direction of travel of the laser oscillation in each waveguide can actually be lossy even to the wave traveling in the desired direction. Similarly, output coupling techniques such as scattering light out of an optical fiber is, necessarily, inherently lossy. Additionally, for ring lasers the optical fibers are bent. This bent or curved fiber and the nature of optical fibers also cause losses. These losses make the optical fiber waveguide a very low Q cavity, at best. Herein, low Q cavity means that the oscillations in the waveguide cavity are far from monochromatic and cannot be used in heterodyne detection techniques to measure the small frequency shift produced by low rotation rates. Also, the light travels around the optical fiber waveguide by internal reflections, and this means that the phase matching requirement will be met for a wide band of wavelengths. The requirement for zero phase shift around the cavity means that the number of wavelengths of the oscillator must be equal to an integer. This means that C/L (the speed of light C divided by the total length of the oscillator L) must be an integer. L is the optical path length, which is different from the geometrial length of the fiber waveguide. Therefore, for each longitudinal mode there is a band of wavelengths which satisfy this requirement for a given length fiber. Thus the fiber waveguide cavity can oscillate over a broad band of frequencies. This again tends to produce an output which is not useful in heterodyne detection at the sensitivities required. Optical fibers also exhibit birefringence. Thus, the optical fibers is a non-reciprocal element contained in the cavity which causes frequency shifts as a function of intensity of the radiation in the cavity. These frequency shifts appear as rotations in a non-rotating system and as errors in a rotating system. This source of errors might be masked by making the band width of the radiation in the cavity much larger than any frequency shifts which may be produced by the optical Kerr effect. However, this again limits the sensitivity and the usefulness of the fiber optic waveguide laser gyro.
All of the above effects place severe requirements on the active medium for an optical fiber laser because the gain curve will have to be maintained above threshold over the entire frequency band of interest. The two active media which may be used in the fiber optic waveguide ring laser gyro are the laser diode and possibly a fiber laser. The laser diode is inherently a broad band source and, when coupled with an inherently low Q cavity, it produces a very insensitive instrument. To overcome the above losses, the laser would have to be pumped hard, which in itself tends to amplify the problem. A fiber laser would be optically pumped. Optical pumps are inherently very broad band, and much of the unwanted, broadband, light would be trapped by the fiber waveguide. This unwanted light becomes noise on the signal which causes an additional limitation on the sensitivity.